(1) Filed of the Invention
This invention relates to an air conditioning system, and more particularly to an automotive air-conditioning system provided with a refrigeration cycle including a variable displacement compressor, a condenser, an expansion valve, and an evaporator.
(2) Description of the Related Art
Conventionally, in an automotive air-conditioning system, a variable displacement-type compressor is employed which is capable of controlling a suction pressure to a constant level depending on a cooling load.
As a variable displacement compressor, a swash plate type is known which has a swash plate disposed in a closed crank chamber and fitted on a rotating shaft receiving a driving force from an engine such that the inclination angle of the swash plate can be changed, and controls a pressure in the crank chamber to thereby change the inclination angle of the swash plate, whereby the amount of stroke of pistons connected to the swash plate is changed to change the displacement of discharged refrigerant. The pressure in the crank chamber is controlled by a capacity control valve. The capacity control valve controls a pressure introduced from a discharge chamber into the crank chamber in response to a suction pressure of the compressor. For example, when a cooling load decreases to make the suction pressure lower than a preset pressure, responsive to the lowered suction pressure, the capacity control valve increases a valve lift thereof, to thereby increase the flow rate of refrigerant introduced from the discharge chamber into the crank chamber. The increase in the differential pressure between the pressure in the crank chamber and the suction pressure cases the inclination angle of the swash plate to be reduced to decrease the stroke of the pistons, whereby the displacement of the compressor is decreased. As a result, the suction pressure is controlled to the preset pressure whereby the vent temperature of the evaporator can be held constant.
In the refrigeration cycle incorporating the above variable displacement compressor capable of controlling the suction pressure to a constant level, as an expansion valve therefor, a cross charge-type is employed. Referring to FIG. 5, in the cross charge, the pressure characteristic of refrigerant in the temperature-sensing chamber of an expansion valve is configured to have a gentler inclination than that of a saturated vapor curve of refrigerant used in the refrigeration cycle. The cross charge is attained by filling the temperature-sensing chamber of the expansion valve with a gas different from refrigerant used in the refrigeration cycle. By utilizing the cross charge, during low load operation in which refrigerant at an outlet of the evaporator has a low temperature, a pressure in the temperature-sensing chamber is higher than the saturated vapor curve, and hence the refrigerant at the outlet of the evaporator is placed in a state not completely evaporated, and returned to the compressor with liquid contained therein. The refrigerant includes lubricating oil for the compressor, so that when the variable displacement compressor is operating with a small capacity, the liquid returned is made use of to compensate for reduction of returned oil due to a decrease in the circulating amount of the refrigerant.
In the cross charge-type expansion valve, however, when the cooling load is low, liquid is returned to the variable displacement compressor, thereby degrading cooling efficiency, whereas during high load operation in which refrigerant at the outlet of the evaporator has a high temperature, the pressure in the temperature-sensing chamber is hard to be raised, and a superheat degree SH becomes too large, which makes it difficult to make the superheat properly balanced.
On the other hand, an air conditioning system is disclosed in Japanese Unexamined Patent Publication No. 2001-133053, which employs, as a variable displacement compressor, a compressor for controlling the flow rate of refrigerant discharged therefrom to a fixed flow rate set by an external signal, and an expansion valve of a normal charge type. According to this air conditioning system, since the variable displacement compressor is formed by a flow rate control-type compressor, it is possible to control the variable displacement compressor such that it causes refrigerant to flow at a flow rate required for circulation of oil during low load operation of the compressor, and since the expansion valve is formed by an expansion valve of the normal charge type, it is possible to hold refrigerant at the outlet of the evaporator in a state superheated to a predetermined superheat SH even during the low load operation, thereby making it possible to maintain a high cooling efficiency of the system.